The invention relates to mobile communication, and more particularly, to telephone transmission between a branch telephone device equipped on a steering wheel and a mobile telephony station on a load-vehicle.
Among the parts on a vehicle, a steering wheel is located nearest to a driver and within easy access of the driver's hands. Therefore, in order to improve the operability of a branch telephone for a driver it is preferable that a control board having key switches including dialling key switches and a hook switch is mounted at the center of the steering wheel.
However, a steering mechanism for transmitting a rotation of the steering wheel to a steering shaft, has a complex structure, so it is difficult to make the wiring of signal cables for connecting the control board mounted at the center of the steering wheel (referred to as a dialling control board hereinafter) to an electric controller (mobile telephony station) fixed on the vehicle. According to a prior art system, the dialling control board is fixed on the steering wheel (refer to Japanese Patent Publication No. 1972/1979). This makes relatively annoying the operation of the dialling key switches on the dialling control board, because the dialling control board turns with the rotation of the steering wheel. According to another prior art system, the dialling control board is fixed on an inner panel at the front of a driver seat. In this case, dialling operation is relatively troublesome since the dialling key switches are remote from the steering wheel. Therefore, it is preferable to equip the dialling control board on the steering wheel in a floating and hence stationary state with respect to the steering wheel. Additionally, it is preferable to arrange a microphone for transmitting a driver's voice near the lips of the driver to decrease intermixture of noise and echo in the load-vehicle.